A conventional electromagnetic clutch includes a ring-shaped rotor driven through a belt by a driving source such as a car engine, an armature disposed to face the end plate of the rotor with a gap therebetween and an electromagnetic coil generating an electromagnetic circuit including the rotor and the armature to attract the armature into contact with the rotor. In this way, the armature rotates together with the rotor and rotational force is transmitted from the engine to an air conditioning unit.
The above rotor is customarily pre-formed by forging, and is then finished by a turning process (this process is disclosed in Japanese Utility Model Publication No. 57-39,639). However, when a rotor is finished by a turning process, a great volume of wast material is generated. The process is also complicated and therefore expensive.
Another method for producing a rotor is disclosed in Japanese Patent Application Laid Open No. 54-14,364 which shows a ring-shaped pulley disposed on the outer peripheral surface of a rotor. The pulley is first formed in a V-shape or W-shape and then disposed on the outer surface of the rotor. Thereafter, the pulley is attached to the rotor by welding or fitting. This step of attaching the pulley to the rotor, required by the separate construction of each, is complicated and therefore reduces manufacturing performance. The cost of the equipment to perform this type of manufacturing process is high, since particular equipment is needed for the welding or fitting steps.